The invention relates to a communication system in which interconnection between terminal equipments capable for both voice and data service consist of at least one connection leg, which is rate-adapted in compliance with the CCITT recommendation V.110 or in a like manner and attached to V.24 interfaces, and to a method for controlling such a connection.
In a communication system of this type, the terminal equipments may comprise a data terminal adapted for a V.110 rate-adapted full-duplex connection by a terminal adapter, and a telephone handset which is, for instance, connected to a network termination or integrated in the data terminal for telephone conversation.
Examples of terminal equipments suitable for both data and voice service modes are the Telefax Group 3 terminal equipment and a personal computer telecommunication software which are adapted by a terminal adapter e.g. for an ISDN V.110 rate-adapted 64 kbit/s UDI bearer service or a GSM PLMN 9.6/4.8/2.4 kbit/s UDI bearer service. The terminal equipment may be full-duplex or half-duplex, and it comprises a standard or non-standard session protocol corresponding to the OSI layers 4 to 7 for the management and use of the voice/data service modes. The network termination equipment may thus be NT in ISDN or MT2 in GSM PLMN, for instance.
The terminal equipments of the Telefax Group 3 have not previously comprised a V.24 interface; instead, they have always utilized a modem interface and a 2-wire connection (modem connection through a telephone network, for instance). As the same transmission connection has thereby been able to transmit both voice and baseband modem signalling, the terminal equipments have been able to perform the transition from data service mode to voice service mode, for instance, in compliance with the applied management protocol of the services of the OSI layers 4 to 7 (e.g. CCITT T.30 or T.4) without any need to indicate the change of the service mode to the used transmission network or without any need for the transmission network to react to the change of service mode by somehow altering the configuration of the transmission connection.
Communication systems utilizing V.110 connections have the drawback that although the terminal equipments comprise a handset for telephone service, the V.110 rate-adapted transmission path is not able to transfer voice. However, the use of telephone service during a call is a normal function in the Telefax Group 3, for instance. Similarly, when receiving an incoming data call by a personal computer, it is necessary to answer the call by speech to enable the terminal user to start an appropriate application.
ISDN and GSM PLMN subscriber signallings will contain an In-Call Modification procedure, by which the speech connection can, if required, be changed into a V.110 rate-adapted data connection, and vice versa, an unlimited number of times by replacing the PCM coding with the UDI coding, and vice versa.
However, it has not been specified how the communication network could react to a service mode change performed within the OSI layers 4 to 7 between terminal equipments designed for modem connections, and as a result of it take the necessary steps to adapt the network for the new service mode e.g. by starting the In-Call Modification procedure to change a V.110 rate-adapted leg in the signal path into a speech connection.
Generally speaking, the problem is that an application utilizing a V.110 leg, such as a telefax service, and associated adapting means signal between each other functions which are significant for the call control (change of service mode, change of transmission rate, release of call). However, the call control is assigned to the network termination e.g. in the mobile telephone of the GSM system.